Angle of attack, or AOA, which is the angle at which the air hits the wing of an aircraft, or the angle α between the airfoil's reference line and the oncoming air flow, is an important and useful concept in controlling an aircraft's critical performance. For example, a stall occurs at a particular angle of attack; the smallest power-off descent rate occurs at a particular angle of attack; the best power-off glide ratio occurs at a particular angle of attack; the recommended approach speed is related to the angle of attack; the best rate of climb occurs at a particular angle of attack; and the best angle of climb occurs at a particular angle of attack.
The so called critical angle of attack is the angle of attack which produces the maximum lift coefficient and is also known as the stall angle of attack. Below the critical angle of attack, as the angle of attack increases, the coefficient of lift increases, while above the critical angle of attack, as the angle of attack increases, the air begins to flow less smoothly over the upper surface of the airfoil and begins to separate from the upper surface. Above this critical angle of attack, the aircraft is said to be in a stall. The airspeed at which the aircraft stalls normally varies with various parameters associated with the aircraft itself, such as the weight of the aircraft, the load factor, the center of gravity and other factors; however, the aircraft always stalls at the same critical angle of attack.
Because of the critical nature of angle of attack in flying the aircraft, particularly in commercial aircraft, various types of instrumentation have been developed over the years to indicate to the pilot what the angle of attack of the aircraft is at various critical points along the flight path of the aircraft above, below, and at the critical angle of attack. This instrumentation has generally fallen into two categories, namely non-normalized body angle of attack indicators, such as disclosed in U.S. Pat. No. 6,131,055, and normalized angle of attack indicators, such as disclosed in U.S. Pat. No. 6,271,769, with each type of system having its own benefits and drawbacks. For example, indicating systems employing non-normalized body angle of attack, a key component of the angle of attack gauge, approach angle of attack, slides up and down a scale based on flap position rather then being at a more readily discernible fixed location, whereas indicating systems employing normalized angle of attack, variations in Mach number can potentially result in misleading readings. Since one key purpose of these angle of attack indicating systems is to provide flight crews with so called quick glance awareness to the proximity of an unsafe operating speed, the presence of either of these disadvantages at a critical time is highly undesirable.
These disadvantages are overcome by the present invention which provides a composite display of both the body angle of attack in a readily discernible digital display adjacent the normalized angle of attack display, thereby retaining the benefits of a body angle of attack gauge while still providing the simplicity of a normalized angle of attack gauge with fixed key reference points.